The present invention relates to a rotary magnetic field type stirrer placed in the secondary cooling zone of a continuous casting machine for stirring the unsolidified part in the interior of the cast strand.
In manufacturing cast strands by a continuous casting method, the casting conditions are strictly controlled in order to prevent the occurrence of internal defects in the cast strands such as segregation, shrinkage cavities and coarse crystals. As a matter of fact, however, the occurrence of these defects cannot be completely prevented by such controls alone. It is already publicly known that, generally speaking, it is not advisable to make cross sectional dimensions of the cast strand smaller, since it increases internal defects and very much reduces the forging effect of the subsequent process of hot working. In consequence, the measure ordinarily taken for the elimination of internal defects is the adoption of a method based on the fact, that a cast strand having a larger cross section is made by continuous casting and then subjected to billeting to manufacture billets or other semi-manufactured products. In this case, however, the omission of billeting, which is one of the major objectives of the continuous casting technique, is not realized. Another measure taken against the occurrence of internal defects is the so-called strand reduction method in which a rolling mill is put in the continuous casting process to decrease such defects as shrinkage cavities, porosity, etc. by reduction on the cast strand immediately after solidification. In this method, however, no improvement is brought about at all with respect to segregation; it is difficult to apply a proper reduction at a proper time in accordance with varying casting conditions; and there is a tendency that internal cracks are liable to take place. Furthermore, with some types of steel such as high carbon steel which has a broad range of solidification temperatures, there is a broad range of co-existence of solid and liquid phases of different constituents, so that segregation of a special kind may be induced by the extraction phenomenon due to the reduction.
Since the patents to Junghans (DP 902434 and 911425), a number of suggestions and proposals have been made concerning the utilization of electromagnetic induction for the purpose of controlling casting conditions in continuous casting. The present applicants, in their previous inventions (Japanese Patent Applications No. 73407/1972 and No. 2301/1973), have shown that the two-pole rotary magnetic field method can effect efficient stirring of the liquid core of a cast strand and that the internal quality of cast strands is improved by this electromagnetic stirring in the secondary cooling zone. That is to say, in the case of a rotary magnetic field having three or more poles the magnetic flux in the peripheral part of the inner space of the strand becomes stronger if the number of poles increases, but the flux in the central part of the magnetic field becomes weaker, so that the electromagnetic force becomes quite insufficient for penetrating the solidified shell of the cast strand and stirring its liquid core when used in continuous casting. Poppmeier (Journal of Metals, 1966.10), who devised a three-pole rotary magnetic field device, anticipated that it might be effective to stir the molten core in the secondary cooling zone where the solidified shell has acquired some degree of thickness, but nevertheless, as it was impossible to stir it, he placed an electromagnetic coil near the surface of molten metal in the mold where stirring was easy. As a result, he was successful in improving the internal quality of cast strands, but at the same time created a serious problem. That is to say, as the surface of the molten metal in the mold makes a rotating movement, non-metallic scum, slag, etc. floating on the surface of the liquid get together in the central part of the eddy or whirlpool because of the difference in density between the metal and scums, and are brought by the casting stream into the interior of the cast strand. Thus a drawback took place in that a part of the non-metallic scum, slag, etc., which are in the form of comparatively small particles, will not come out of the interior of the cast strand but results in an increased number of non-metallic inclusions in the interior of the cast strand.
In the previous invention, on the other hand, the principle of a two-pole rotary magnetic field enabled the electromagnetic force to act on the molten core even if the shell thickness was great, so that it was possible to install this type of stirrer at any location on a continuous casting machine. If this rotary magnetic field type stirrer is installed in the secondary cooling zone instead of in the neighborhood of the surface of molten metal in the mold where the shell thickness is small, the solidification front there consists of brittle protrusions, which are easily broken into numerous nuclei by stirring and grow into equi-axial crystals, since the molten core in that zone has already been cooled to a considerable degree.
In consequence of this, the strand has a fine and homogeneous structure, the occurrence of shrinkage cavities and reducing segregation being prevented. However, the device of the previous invention (Japanese Patent Application No. 73407/1972) which was a rotary magnetic field stirrer of the type having a two-pole three-phase induction motor stator, had the following disadvantage. From the viewpoint of performance, the distance between poles of the rotary magnetic field type stirrer is long because it is determined by the diagonal dimension of the cross section of the cast strand, so that the magnetic resistance between the poles inevitably becomes great and makes it difficult to obtain sufficient magnetic flux, especially for use on a large section strand. Because of this reason, the apparatus and power source are naturally required to be of a large output. Besides, when installing this stirrer in a continuous casting machine, it is impossible to carry out stirring two or more times by installing two or more such stirrers near to each other. It is thus difficult to realize an optimum stirring condition. From the viewpoint of installation cost and maintenance, 1.) it is necessary to provide as many rotary magnetic field stirrers as possible to correspond to the number of different types and dimensions of cast strand cross sections and a large amount of equipment cost is required also for keeping spare units on hand: 2.) because of the construction of the stirrer, the breaking of a part of the coils impairs the functioning of the whole stirrer, thus requiring a high cost of maintenance for the stirrer. This stirrer of the prior art thus had such weak points.
With the stirrer of the previous invention (Japanese Patent Application No. 23010/1973), which also related to a two-pole single phase alternating current rotary magnetic field type stirrer, it was possible to conduct stirring only once with one stirrer.